Oxygen levels, hypoxemia & O2 terminology

Oxygen is an essential medicine that is needed treat (e.g. pneumonia) or safely manage (e.g. safe surgery and anesthesia) patients with a wide range of conditions. Nonetheless, oxygen is not universally accessible in all health systems and for all patients. In this article we discuss some of the fundamental terminology related to oxygen.

Low blood oxygen levels is termed hypoxemia. Acute hypoxemia can cause tissue hypoxia (low oxygen levels at the cellular/tissue level) which can lead to organ dysfunction and death.

While there is no universal definition for hypoxemia, the most frequently used definition is a functional oxygen saturation (sO2) of <90% or an arterial partial pressure of oxygen of <60 mmHg. Below we define these terms.

Functional Oxygen Saturation (sO2) is the percent of hemoglobin which is capable of transporting oxygen Use the content level slider at the top left to view intermediate and advanced versions of this article for illustrations of functional and fractional saturation.. Functional oxygen saturation can be measured non-invasive by a pulse oximeter and is termed SpO2, or can be measured with a blood gas analysis by a co-oximeter and is termed SaO2 for arterial functional saturation, SvO2 for venous saturation, ScvO2 for central venous saturation an so on.

Check out more information on the clinical use of SpO2 and SpO2 clinical targets.

Fraction of inspired oxygen (FiO2) is the proportion of inhaled gas that is oxygen. This is generally expressed as a decimal. For example, room air is 0.21 FiO2, whereas pure oxygen (i.e. no nitrogen) is 1.0 FiO2.

Read more about titration of FiO2

The optimal target for oxygen saturation (SpO2) in patients with acute hypoxemic respiratory failure is unknown. Hypoxemia causes pulmonary vasoconstriction and pulmonary hypertension in its chronic form, and death when it is acute and severe. Hyperoxemia also causes physiologic disturbances, through toxic reactive oxygen species and absorption atelectasis (Angus et al).

The World Health Organization (WHO) interim guidance for patients with hypoxemic respiratory failure due to COVID-19 suggests an initial SpO2 target >94% for stabilization, then >90% for non-pregnant patients and 92-95% for pregnant patients, once stable (WHO SARI Toolkit). The summary of evidence below suggests another reasonable target might be SpO2 90-96%, and perhaps 92-96% in settings with only intermittent pulse oximetry monitoring, or in patients with darker skin pigmentation.

According to the WHO, medicinal oxygen contains >82% pure oxygen, is free from contamination and must come from an oil-free compressor. Oxygen was added to the World Health Organization (WHO) list of essential medicines (EML) in 1979 for use in anesthesia, and was amended in 2017 for treating hypoxemia. There is ongoing debate and lack of consensus around the minimum required oxygen concentration for clinical/medical grade oxygen sources.

While the European Pharmacopoeia and the US Pharmacopeia include “Oxygen 93%,” the International Pharmacopeia 10th Edition 2020 contains only “Oxygen 99%.” The 11th edition is currently being revised with consideration to also add 93% by WHO. Of note, there is the International Pharmacopoeia, regional pharmacopoeias (e.g. European), and national pharmacopoeias (e.g. United States Pharmacopeia), each with defined values for medical grade oxygen concentration and purity.

It is widely considered that medicinal oxygen supply should contain >93% oxygen purity from a certified medical oxygen supplier or source. Of note, it is common that oxygen concentration from various sources (e.g. PSA plants, cylinders, portable concentrators) can vary widely and facility level quality control with oxygen analyzers is always recommended.

(Read more on oxygen supply types)

• Winter, Bruce, Bach, Jay, Eger. “The anesthetic effect of air at atmospheric pressure.” Anesthesiology. 1975
• Parker C, Snowdon S. “Predicted and measured oxygen concentrations in the circle system using low fresh gas flows with oxygen supplied by an oxygen concentrator,” British Journal of Anaesthesia, 1988 Oct;61(4):397-402
• Angus DC. Oxygen Therapy for the Critically Ill. N Engl J Med 2020;382:1054-6.
• World Health Organization. Clinical management of severe acute respiratory infection (SARI) when COVID-19 disease is suspected: Interim guidance. 2020 March 13.
• Helmerhorst HJ, Schultz MJ, van der Voort PH, Bosman RJ, Juffermans NP, de Wilde RB, van den Akker-van Marle ME, van Bodegom-Vos L, de Vries M, Eslami S, de Keizer NF, Abu-Hanna A, van Westerloo DJ, de Jonge E. Effectiveness and Clinical Outcomes of a Two-Step Implementation of Conservative Oxygenation Targets in Critically Ill Patients: A Before and After Trial. Crit Care Med 2016;44:554-63.
• Girardis M, Busani S, Damiani E, Donati A, Rinaldi L, Marudi A, Morelli A, Antonelli M, Singer M. Effect of Conservative vs Conventional Oxygen Therapy on Mortality Among Patients in an Intensive Care Unit: The Oxygen-ICU Randomized Clinical Trial. Jama 2016;316:1583-9.
• Barrot L, Asfar P, Mauny F, Winiszewski H, Montini F, Badie J, Quenot JP, Pili-Floury S, Bouhemad B, Louis G, Souweine B, Collange O, Pottecher J, Levy B, Puyraveau M, Vettoretti L, Constantin JM, Capellier G. Liberal or Conservative Oxygen Therapy for Acute Respiratory Distress Syndrome. N Engl J Med 2020;382:999-1008.
• Mackle D, Bellomo R, Bailey M, Beasley R, Deane A, Eastwood G, Finfer S, Freebairn R, King V, Linke N, Litton E, McArthur C, McGuinness S, Panwar R, Young P. Conservative Oxygen Therapy during Mechanical Ventilation in the ICU. N Engl J Med 2020;382:989-98.
• Schjørring OL, Klitgaard TL, Perner A, Wetterslev J, Lange T, Siegemund M, Bäcklund M, Keus F, Laake JH, Morgan M, Thormar KM, Rosborg SA, Bisgaard J, Erntgaard AES, Lynnerup AH, Pedersen RL, Crescioli E, Gielstrup TC, Behzadi MT, Poulsen LM, Estrup S, Laigaard JP, Andersen C, Mortensen CB, Brand BA, White J, Jarnvig IL, Møller MH, Quist L, Bestle MH, Schønemann-Lund M, Kamper MK, Hindborg M, Hollinger A, Gebhard CE, Zellweger N, Meyhoff CS, Hjort M, Bech LK, Grøfte T, Bundgaard H, Østergaard LHM, Thyø MA, Hildebrandt T, Uslu B, Sølling CG, Møller-Nielsen N, Brøchner AC, Borup M, Okkonen M, Dieperink W, Pedersen UG, Andreasen AS, Buus L, Aslam TN, Winding RR, Schefold JC, Thorup SB, Iversen SA, Engstrøm J, Kjær MN, Rasmussen BS. Lower or Higher Oxygenation Targets for Acute Hypoxemic Respiratory Failure. N Engl J Med 2021.
• Sutherland T, Moriau V, Niyonzima JM, Mueller A, Kabeja L, Twagirumugabe T, Rosenberg N, Umuhire OF, Talmor DS, Riviello ED. The “Just Right” Amount of Oxygen. Improving Oxygen Use in a Rwandan Emergency Department. Ann Am Thorac Soc 2019;16:1138-42.
• Lipnick MS, Feiner JR, Au P, Bernstein M, Bickler PE. The Accuracy of 6 Inexpensive Pulse Oximeters Not Cleared by the Food and Drug Administration: The Possible Global Public Health Implications. Anesth Analg 2016;123:338-45.
• Sjoding MW, Dickson RP, Iwashyna TJ, Gay SE, Valley TS. Racial Bias in Pulse Oximetry Measurement. N Engl J Med 2020;383:2477-8.
• WHO Good manufacturing practices for medicinal gases. WHO. 2021
• Medicinal oxygen working document. WHO. 2022
• International Pharmacopoeia, 10th Edition, 2020
• WHO Model Lists of Essential Medicine, 2022